Planetary Volcanism 843
FIGURE 12 A Hawaii eruption
from the Pu’u ‘O’o vent in Hawaii
showing a convecting cloud of gas
and small particles in the
atmosphere above the 300 m high
lava fountain (commonly termed
fire fountain) of coarser basaltic
pyroclasts. (Photograph by P. J.
Mouginis-Mark.)cone, cinder cone, or scoria cone, the term used depending
on the sizes of the pyroclasts involved, ash being smallest.
Such pyroclastic cones are commonly asymmetric owing to
the influence of the prevailing wind.
Atmospheric gases are entrained into the edge of the fire
fountain and heated by contact with the hot pyroclasts and
mixing with the hot magmatic gas. In this way, a convecting
gas cloud is formed over the upper part of the fountain,
and this gas entrains the smallest pyroclasts so that they
take part fully in the convective motion. The whole cloud
spreads downwind and cools, and eventually the pyroclasts
are released again to form a layer on the ground, the smallest
particles being deposited at the greatest distances from the
vent. This whole process, involving formation of lava flows
and pyroclastic deposits at the same time, is called Hawaiian
eruptive activity (Fig. 12). This style of activity should cer-
tainly have occurred on Mars, but may be suppressed in
basaltic magmas on Venus by the high atmospheric pres-
sure, especially in lowland areas, unless, as noted earlier,
magma volatile contents are several times higher than is
common on Earth.
Figure 13 shows qualitatively how the combination of
erupting mass flux and magma gas content in a Hawaiian
eruption on Earth determines the nature and size of the
possible products: a liquid lava pond at the vent that di-
rectly feeds lava flows; a pile of slightly cooled pyroclasts
accumulating fast enough to weld together and form a “root-
less” lava flow; a cone in which almost all of the pyroclasts
are welded together; or a cone formed from pyroclasts that
have had time to cool while in flight so that none, or only a
few, weld on landing. Attempts have been made to quantify
the results in Fig. 13 and extend them to other planetary
environments. These results confirm that hot lava ponds
around vents on Earth are expected to be no more than a
few tens of meters wide even at very high mass eruption
rates. On the Moon, the greater gas expansion due to the
lack of an atmosphere causes very thorough disruption of
the magma (even at the low gas contents implied by analysis
of theApollosamples) and gives the released volcanic gas
a high speed. This, together with the lower gravity, allows
greater dispersal of pyroclasts of all sizes and provides an
explanation of the 100–300 km wide dark mantle depositsFIGURE 13 Schematic indication of the relative influences of
the volatile content and the volume eruption rate of magma on
the dispersal and thermal state of pyroclastic material produced
in explosive eruptions. (Reprinted from Fig. 5 in theJournal of
Volcanology and Geothermal Research, Vol. 37, J. W. Head and
L. Wilson, Basaltic pyroclastic eruptions: Influence of gas-release
patterns and volume fluxes on fountain structure, and the
formation of cinder cones, spatter cones, rootless flows, lava
ponds and lava flows, pp. 261–271,©C1989, with kind permission
of Elsevier Science–NL, Sara Burgerhartstraat 25, 1025 KV
Amsterdam, The Netherlands.)